Ioannis Bitsanis - Academia.edu (original) (raw)
Papers by Ioannis Bitsanis
Journal of Polymer Science Part B, Nov 10, 2009
When a polymer chain in solution interacts with an atomically smooth solid substrate, its conform... more When a polymer chain in solution interacts with an atomically smooth solid substrate, its conformational properties are strongly modified and deviate substantially from those of chains in bulk. In this work, the interplay of two competing transitions that affect the conformations of polymer chains near an energetically attractive surface is studied by means of Monte Carlo simulations on a cubic lattice. The transition from an extended to a compact conformation of a polymer chain near an attractive wall, as solubility deteriorates, exhibits characteristics akin to the “coil‐to‐globule” transition in bulk. An effective θ‐temperature is determined. Its role as the transition point is confirmed in a variety of ways. The nature of the coil‐to‐compact transition is not qualitatively different from that in the bulk. Adsorbed polymer chains may assume “globular” or “pancake” configurations depending on the competition among adsorption strength, cohesive energy, and entropy. In a very relevant range of conditions, the dependence of the adsorbate thickness on chain‐length is intermediate between that of 3‐d (“semidroplets”) and 2‐d (“pancake”) objects. The focus of this study is on rather long polymer chains. Several crucial features of the transitions of the adsorbed chains are N‐dependent and various aspects of the adsorption and “dissolution” process are manifested clearly only at the “long chain” limit. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2462–2476, 2009
Journal of Polymer Science, 2009
Monte Carlo computer simulations of end-tethered chains grafted onto a hard wall have been perfor... more Monte Carlo computer simulations of end-tethered chains grafted onto a hard wall have been performed. The chains were modeled as self-avoiding chains on a cubic lattice at athermal solvent conditions. The simulations spanned a wide range of chain lengths, N (100-1000, i.e., up to molecular weights of a few hundred thousands), and anchoring densities, σ (2 x 10⁻⁴ to 0.4), to properly chart the relevant parameter space. It is shown that the reduced surface coverage σ* = σπRg ² is the most appropriate variable that quantitatively determines the mushroom, overlapping mushroom and brush regimes, where Rg is the radius of gyration of a free chain in solution. The simulation data are analyzed to determine the conformational characteristics and shape of the anchored chains and to compare them with the predictions of the analytical self consistent field theory. The strong stretching limit of the theoretical predictions is obtained only for σ* > 8.
Macromolecules, Sep 1, 1988
We report the results of Brownian dynamics simulations for solutions of rigid rodlike polymers wi... more We report the results of Brownian dynamics simulations for solutions of rigid rodlike polymers with an aspect ratio of 50 at high concentrations (70-150 rods/13, where L is the molecular length). Many-body contributions to the static properties were found to be much weaker than direct two-body interactions up to the highest concentration simulated; therefore the local structure was mostly determined by independent binary interactions. Furthermore, the solvent-polymer impacts in between rod-rod collisions killed most of the temporal correlation of successive rod-rod collisions reducing in this way a highly correlated rod-rod collision sequence to a series of almostuncorrelated binary collisions. At sufficiently high concentrations (higher than 90 rods/13) the translational diffusivity normal to the rod axis froze, which proves the existence of completely closed cages. In this concentration regime our simulation results for the rotational diffusivity fall between the predictions of the Doi-Edwards theory for rods with finite diameter and those of Fixman's model. The discrepancy between Fixman's model and our simulation results at high concentrations probably originates from the very sharp decrease of the equilibrium cage size with concentration, which could make the short time relaxation mechanism introduced by Fixman inefficient.
arXiv (Cornell University), Dec 12, 2013
I. G. Aviziotis et al. range of bistability is determined for different sizes of cell populations... more I. G. Aviziotis et al. range of bistability is determined for different sizes of cell populations. The results are compared with the deterministic cell population balance (CPB) model, which is valid for large populations, and we demonstrate the increased effect of stochasticity in small size populations with asymmetric partitioning mechanisms.
Acs Symposium Series, Oct 22, 1987
Journal of Chemical Physics, Mar 15, 1990
We report a molecular dynamics study of the structure and single-particle dynamics of mixtures of... more We report a molecular dynamics study of the structure and single-particle dynamics of mixtures of a protic (ethylammonium nitrate) and an aprotic (1-butyl-3-methylimidazolium hexaflurophosphate [BMIM][PF 6 ]) room-temperature ionic liquids doped with magnesium and calcium salts with a common anion at 298.15 K and 1 atm. The solvation of these divalent cations in dense ionic environments is analyzed by means of apparent molar volumes of the mixtures, radial distribution functions, and coordination numbers. For the protic mixtures, the effect of salt concentration on the network of hydrogen bonds is also considered. Moreover, single-particle dynamics of the salt cations is studied by means of their velocity autocorrelation functions and vibrational densities of states, explicitly analyzing the influence of salt concentration, and cation charge and mass on these magnitudes. The effect of the valency of the salt cation on these properties is considered comparing the results with those for the corresponding mixtures with lithium salts. We found that the main structural and dynamic features of the local solvation of divalent cations in ionic liquids are similar to those of monovalent salts, with cations being localized in the polar nanoregions of the bulk mixture coordinated in monodentate and bidentate coordination modes by the [NO 3 ] − and [PF 6 ] − anions. However, stronger electrostatic correlations of these polar nanoregions than in mixtures with salts with monovalent cations are found. The vibrational modes of the ionic liquid (IL) are seen to be scarcely affected by the addition of the salt, and the effect of mass and charge on the vibrational densities of states of the dissolved cations is reported. Cation mass is seen to exert a deeper influence than charge on the low-frequency vibrational spectra, giving a red shift of the vibrational modes and a virtual suppression of the higher energy vibrational modes for the heavier Ca 2+ cations. No qualitative difference with monovalent cations was found in what solvation is concerned, which suggests that no enhanced reduction of the mobility of these cations and their complexes in ILs respective to those of monovalent cations is to be expected.
Macromolecules, 2018
Monte Carlo computer simulations were utilized to probe the behavior of homopolymer droplets adso... more Monte Carlo computer simulations were utilized to probe the behavior of homopolymer droplets adsorbed at solid surfaces as a function of the number of chains making up the droplets and varying droplet sizes. The wetting behavior is quantified via the ratio of the perpendicular to the parallel component of the effective radii of gyration of the droplets and is analyzed further in terms of the adsorption behavior of the polymer chains and the monomers that constitute the droplets. This analysis is complemented by an account of the shape of the droplets in terms of the principal moments of the radius of gyration tensor. Single-chain droplets are found to lie flatter and wet the substrate more than chemically identical multi-chain droplets, which attain a more globular shape and wet the substrate less. The simulation findings are in good agreement with atomic force microscopy (AFM) experiments. The present investigation illustrates a marked dependence of wetting and adsorption on certain structural arrangements and propose this dependence as a technique through which polymer wetting may be tuned.
Submitted for the MAR09 Meeting of The American Physical Society Collapse Transitions in Thermose... more Submitted for the MAR09 Meeting of The American Physical Society Collapse Transitions in Thermosensitive Alternating Copolymers: A Monte Carlo Study IOANNIS BITSANIS, ANASTASIA RISSANOU, FORTHIESL, Heraklion, Greece, STANISLAV BUROV, St. Petersburgh Univ., Russia, EVEANGELOS MANIAS, Penn State Univ., PA USA — Alternating copolymers are expected to exhibit a rich transition behavior in selective solvents with implications in biology and the design of thermo and pH-sensitive materials. We studied transitions of model alternating copolymers of the type (AAA...)n1(BBB...)n2, in selective solvents by MC simulations. Results showed that the eminent factor, controlling response to external stimuli, is co-polymer’s chemical composition.. We focused on the extreme case of a single polymer chain of N = 1000 units, distributed equally in alternate blocks of n1= n2 =100 units (Aand Bblocks). The solvent was quite selective, i.e. good for 5 100-A-blocks, whereas the 5 100 B-blocks were quite ins...
Submitted for the MAR09 Meeting of The American Physical Society Phase Equilibrium of Size-Disper... more Submitted for the MAR09 Meeting of The American Physical Society Phase Equilibrium of Size-Dispersed Colloid Systems with soft pair interactions: A Monte Carlo Study IOANNIS BITSANIS, FORTH-IESL, Heraklion, Greece, IOANNIS ECONOMOY, MARIANNA YIANNOURAKOU, NRCPS “Demokritos”, Ag. Paraskevi, Greece — We have studied the solid-fluid coexistence for systems of polydisperse soft spheres that interact via power-law potentials. We employed isobaric semi-grand ensemble simulations. Gibbs-Duhem integration traced the coexistence pressure as a function of the variance of the imposed activity distribution. Fluid-solid coexistence densities were determined to be monotonically increasing functions of the breadth of particle size dispersity. Our simulations testified to the existence of terminal diameter dispersity, i.e a dispersity above which there can be no amorphous-crystalline phase coexistence. At the terminus, size-dispersity increases from 5.8% to 6.7% for the crystalline phase as pair in...
MRS Proceedings, 2002
Molecular dynamics simulations were used to study the interlayer structure and dynamics of polyst... more Molecular dynamics simulations were used to study the interlayer structure and dynamics of polystyrene (PS) and polystyrene-polyisoprene (PS-PI) block copolymers intercalated in organically modified layered silicates. In the case of PS the polymer chains displace the aliphatic surfactant chains and reside adjacent to the silicate layers. The electrostatic interactions between the aromatic rings on the PS chains and the silicate surface drive the intercalation of the polymer into the host galleries. PI, which lacks such electrostatic interactions, is immiscible (does not intercalate) with the host. There appears to be a minimum number of PS mers for intercalation of PS-PI copolymers to take place. The intercalated copolymer appears to structure inside the host galleries with the PS mers adjacent to the silicate layers and the corresponding PI away from the surface and towards the middle of the gallery. Using the mean square displacements we find that PS is the least mobile species in...
Thesis University of Minnesota 1989 Source Dissertation Abstracts International Volume 50 07 Section B Page 3038, 1989
AIP Conference Proceedings, 2007
We study the conformational transitions of polymer chains near attractive surfaces using Monte Ca... more We study the conformational transitions of polymer chains near attractive surfaces using Monte Carlo simulations on a simple cubic lattice. The shape of the adsorbed chain is studied as a function of the molecular weight, solvent quality and surface energy. The competition between two transitions induced by varying solvent conditions and the existence of an attractive wall results in various shapes of the adsorbate.
AIP Conference Proceedings, 2007
The symposium focuses on the current state-of-the-art in "Computational Methods in Chemical Engin... more The symposium focuses on the current state-of-the-art in "Computational Methods in Chemical Engineering: Physical Chemistry", as they lead to the next generation of methods, approaches, and research foci in the field. This field encompasses a broad range of problems and approaches: ranging from atomistic quantum mechanical calculation of molecules to computer aided modeling and simulation in the development, integration and optimization of industrial processes. Here we limit the symposium's thematic focus on topics that are under the general scope of "Physical Chemistry", in the broader sense, embracing traditional chemical engineering topics (e.g. thermodynamics, solutions, polymers) and looking forward to research directions where the field is expanding (e.g. life sciences, biological engineering).
AIP Conference Proceedings, 2007
We have studied the solid-fluid coexistence for systems of polydisperse soft spheres with near-Ga... more We have studied the solid-fluid coexistence for systems of polydisperse soft spheres with near-Gaussian diameter distributions that interact via an inverse power potential, the softness of which is being tuned. The study employs simulation in the isobaric semigrand ensemble. Gibbs-Duhem integration is used to trace the coexistence pressure as a function of the variance of the imposed activity distribution. Both
We have studied the solid-fluid coexistence for systems of polydisperse soft spheres that interac... more We have studied the solid-fluid coexistence for systems of polydisperse soft spheres that interact via power-law potentials. We employed isobaric semi-grand ensemble simulations. Gibbs-Duhem integration traced the coexistence pressure as a function of the variance of the imposed activity distribution. Fluid-solid coexistence densities were determined to be monotonically increasing functions of the breadth of particle size dispersity. Our simulations testified
Physical Review A, 1987
The statistical uncertainty in the calculation of velocity autocorrelation functions and self-dif... more The statistical uncertainty in the calculation of velocity autocorrelation functions and self-dift'usion coefficients from molecular dynamics is empirically determined from the spread of the moleculardynamics results for an ensemble of macroscopically identical systems. The "experimental" uncertainties of the velocity-autocorrelation-function values at equilibrium and in the presence of flow agree well with theoretical predictions. The uncertainty of the self-diffusion coefficient is found to decrease as the inverse square root of the averaging time.
Makromolekulare Chemie. Macromolecular Symposia, 1993
We have performed molecular dynamics, and lattice Monte Car10 simulations of polymeric melts in t... more We have performed molecular dynamics, and lattice Monte Car10 simulations of polymeric melts in the vicinity of solid surfaces. The structural features of the solid-melt interface were very simple. The interfacial width was comparable to the segment size. Inside this narrow interface the segment density profile was oscillatory. The density oscillations were much less pronounced than those present at solid-atomic liquid interfaces. On a scale much larger than the segment size, chain conformations were found to be identical with those of ideal chains next to a reflective barrier. In particular, the number of surface-segment contacts scaled like the square root of the molecular weight. Extensive molecular dynamics simulations showed that chain desorption times increase with molecular weight but at a rate much slower than the longest relaxation time of Rouse chains. Therefore, sufficiently long chains desorbed almost freely from the surface despite the presence of attractive surface-segment interactions. A study of chain relaxation dynamics c o n f i i e d that the Rouse modes constitute an appropriate set of normal coordinates for chains in the melt interacting with a solid surface. The effect of the surface on mode relaxation was significant. All relaxation processes of chains located within a couple of radii of gyration from the surface were slowed down considerably. This effect, however was approximately the same for fast and slow modes and independent of molecular weight for sufficiently long chains.
We performed lattice MC simulations of single, flexible, self-avoiding chains in bulk solution^*,... more We performed lattice MC simulations of single, flexible, self-avoiding chains in bulk solution^*, or adsorbed onto a surface, under poor solvent conditions. Our simulations spanned a wide range of chain lengths (N=20-10000) and cohesive energies. The chain length dependence of the chain size in poor solvents was characterized by a wide plateau of almost null growth. This plateau was related with the development of the incipient constant density core. The ``volume approximation'' regime and genuine power law dependence (1/3) was not reached even for the longest chains and poorest solvents studied. Sufficiently long chains became more but not fully spherical and underwent a 2^nd order phase transition. Conformations of the adsorbed chains onto attractive surfaces are not controlled by the bulk theta- temperature, but by a new temperature theta' which depends strongly on the interactions with the surface. The adsorption-desorption transition width is determined by the N-dep...
Dense suspensions of multi-arm star polymers are known to exhibit `liquid-like' ordering due ... more Dense suspensions of multi-arm star polymers are known to exhibit `liquid-like' ordering due to their similarities with colloidal particles. Experimental studies reported a counter-intuitive solidification of these suspensions upon increase of the temperature in marginal solvents( M. Kapnistos, D. Vlassopoulos, G. Fytas, K. Mortensen, G. Fleischer, J. Roovers, Phys. Rev. Lett. 85), 4072 (2000).. Our work investigates by MD simulations the temperature induced changes in colloidal superstructure and star dynamics under marginal solvent conditions. Multi-arm star polymers are modeled as soft spheres interacting via a soft, long ranged potential, derived from polymer physics principles. Simulations show a transition towards a ``glassy'' state at a temperature very close to the one reported experimentally. The features of the transition are consistent with those of ideal glass transitions, as described by ideal Mode Coupling Theory^2 M. Kapnistos, D. Vlassopoulos, G. Fytas, K...
Journal of Polymer Science Part B, Nov 10, 2009
When a polymer chain in solution interacts with an atomically smooth solid substrate, its conform... more When a polymer chain in solution interacts with an atomically smooth solid substrate, its conformational properties are strongly modified and deviate substantially from those of chains in bulk. In this work, the interplay of two competing transitions that affect the conformations of polymer chains near an energetically attractive surface is studied by means of Monte Carlo simulations on a cubic lattice. The transition from an extended to a compact conformation of a polymer chain near an attractive wall, as solubility deteriorates, exhibits characteristics akin to the “coil‐to‐globule” transition in bulk. An effective θ‐temperature is determined. Its role as the transition point is confirmed in a variety of ways. The nature of the coil‐to‐compact transition is not qualitatively different from that in the bulk. Adsorbed polymer chains may assume “globular” or “pancake” configurations depending on the competition among adsorption strength, cohesive energy, and entropy. In a very relevant range of conditions, the dependence of the adsorbate thickness on chain‐length is intermediate between that of 3‐d (“semidroplets”) and 2‐d (“pancake”) objects. The focus of this study is on rather long polymer chains. Several crucial features of the transitions of the adsorbed chains are N‐dependent and various aspects of the adsorption and “dissolution” process are manifested clearly only at the “long chain” limit. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2462–2476, 2009
Journal of Polymer Science, 2009
Monte Carlo computer simulations of end-tethered chains grafted onto a hard wall have been perfor... more Monte Carlo computer simulations of end-tethered chains grafted onto a hard wall have been performed. The chains were modeled as self-avoiding chains on a cubic lattice at athermal solvent conditions. The simulations spanned a wide range of chain lengths, N (100-1000, i.e., up to molecular weights of a few hundred thousands), and anchoring densities, σ (2 x 10⁻⁴ to 0.4), to properly chart the relevant parameter space. It is shown that the reduced surface coverage σ* = σπRg ² is the most appropriate variable that quantitatively determines the mushroom, overlapping mushroom and brush regimes, where Rg is the radius of gyration of a free chain in solution. The simulation data are analyzed to determine the conformational characteristics and shape of the anchored chains and to compare them with the predictions of the analytical self consistent field theory. The strong stretching limit of the theoretical predictions is obtained only for σ* > 8.
Macromolecules, Sep 1, 1988
We report the results of Brownian dynamics simulations for solutions of rigid rodlike polymers wi... more We report the results of Brownian dynamics simulations for solutions of rigid rodlike polymers with an aspect ratio of 50 at high concentrations (70-150 rods/13, where L is the molecular length). Many-body contributions to the static properties were found to be much weaker than direct two-body interactions up to the highest concentration simulated; therefore the local structure was mostly determined by independent binary interactions. Furthermore, the solvent-polymer impacts in between rod-rod collisions killed most of the temporal correlation of successive rod-rod collisions reducing in this way a highly correlated rod-rod collision sequence to a series of almostuncorrelated binary collisions. At sufficiently high concentrations (higher than 90 rods/13) the translational diffusivity normal to the rod axis froze, which proves the existence of completely closed cages. In this concentration regime our simulation results for the rotational diffusivity fall between the predictions of the Doi-Edwards theory for rods with finite diameter and those of Fixman's model. The discrepancy between Fixman's model and our simulation results at high concentrations probably originates from the very sharp decrease of the equilibrium cage size with concentration, which could make the short time relaxation mechanism introduced by Fixman inefficient.
arXiv (Cornell University), Dec 12, 2013
I. G. Aviziotis et al. range of bistability is determined for different sizes of cell populations... more I. G. Aviziotis et al. range of bistability is determined for different sizes of cell populations. The results are compared with the deterministic cell population balance (CPB) model, which is valid for large populations, and we demonstrate the increased effect of stochasticity in small size populations with asymmetric partitioning mechanisms.
Acs Symposium Series, Oct 22, 1987
Journal of Chemical Physics, Mar 15, 1990
We report a molecular dynamics study of the structure and single-particle dynamics of mixtures of... more We report a molecular dynamics study of the structure and single-particle dynamics of mixtures of a protic (ethylammonium nitrate) and an aprotic (1-butyl-3-methylimidazolium hexaflurophosphate [BMIM][PF 6 ]) room-temperature ionic liquids doped with magnesium and calcium salts with a common anion at 298.15 K and 1 atm. The solvation of these divalent cations in dense ionic environments is analyzed by means of apparent molar volumes of the mixtures, radial distribution functions, and coordination numbers. For the protic mixtures, the effect of salt concentration on the network of hydrogen bonds is also considered. Moreover, single-particle dynamics of the salt cations is studied by means of their velocity autocorrelation functions and vibrational densities of states, explicitly analyzing the influence of salt concentration, and cation charge and mass on these magnitudes. The effect of the valency of the salt cation on these properties is considered comparing the results with those for the corresponding mixtures with lithium salts. We found that the main structural and dynamic features of the local solvation of divalent cations in ionic liquids are similar to those of monovalent salts, with cations being localized in the polar nanoregions of the bulk mixture coordinated in monodentate and bidentate coordination modes by the [NO 3 ] − and [PF 6 ] − anions. However, stronger electrostatic correlations of these polar nanoregions than in mixtures with salts with monovalent cations are found. The vibrational modes of the ionic liquid (IL) are seen to be scarcely affected by the addition of the salt, and the effect of mass and charge on the vibrational densities of states of the dissolved cations is reported. Cation mass is seen to exert a deeper influence than charge on the low-frequency vibrational spectra, giving a red shift of the vibrational modes and a virtual suppression of the higher energy vibrational modes for the heavier Ca 2+ cations. No qualitative difference with monovalent cations was found in what solvation is concerned, which suggests that no enhanced reduction of the mobility of these cations and their complexes in ILs respective to those of monovalent cations is to be expected.
Macromolecules, 2018
Monte Carlo computer simulations were utilized to probe the behavior of homopolymer droplets adso... more Monte Carlo computer simulations were utilized to probe the behavior of homopolymer droplets adsorbed at solid surfaces as a function of the number of chains making up the droplets and varying droplet sizes. The wetting behavior is quantified via the ratio of the perpendicular to the parallel component of the effective radii of gyration of the droplets and is analyzed further in terms of the adsorption behavior of the polymer chains and the monomers that constitute the droplets. This analysis is complemented by an account of the shape of the droplets in terms of the principal moments of the radius of gyration tensor. Single-chain droplets are found to lie flatter and wet the substrate more than chemically identical multi-chain droplets, which attain a more globular shape and wet the substrate less. The simulation findings are in good agreement with atomic force microscopy (AFM) experiments. The present investigation illustrates a marked dependence of wetting and adsorption on certain structural arrangements and propose this dependence as a technique through which polymer wetting may be tuned.
Submitted for the MAR09 Meeting of The American Physical Society Collapse Transitions in Thermose... more Submitted for the MAR09 Meeting of The American Physical Society Collapse Transitions in Thermosensitive Alternating Copolymers: A Monte Carlo Study IOANNIS BITSANIS, ANASTASIA RISSANOU, FORTHIESL, Heraklion, Greece, STANISLAV BUROV, St. Petersburgh Univ., Russia, EVEANGELOS MANIAS, Penn State Univ., PA USA — Alternating copolymers are expected to exhibit a rich transition behavior in selective solvents with implications in biology and the design of thermo and pH-sensitive materials. We studied transitions of model alternating copolymers of the type (AAA...)n1(BBB...)n2, in selective solvents by MC simulations. Results showed that the eminent factor, controlling response to external stimuli, is co-polymer’s chemical composition.. We focused on the extreme case of a single polymer chain of N = 1000 units, distributed equally in alternate blocks of n1= n2 =100 units (Aand Bblocks). The solvent was quite selective, i.e. good for 5 100-A-blocks, whereas the 5 100 B-blocks were quite ins...
Submitted for the MAR09 Meeting of The American Physical Society Phase Equilibrium of Size-Disper... more Submitted for the MAR09 Meeting of The American Physical Society Phase Equilibrium of Size-Dispersed Colloid Systems with soft pair interactions: A Monte Carlo Study IOANNIS BITSANIS, FORTH-IESL, Heraklion, Greece, IOANNIS ECONOMOY, MARIANNA YIANNOURAKOU, NRCPS “Demokritos”, Ag. Paraskevi, Greece — We have studied the solid-fluid coexistence for systems of polydisperse soft spheres that interact via power-law potentials. We employed isobaric semi-grand ensemble simulations. Gibbs-Duhem integration traced the coexistence pressure as a function of the variance of the imposed activity distribution. Fluid-solid coexistence densities were determined to be monotonically increasing functions of the breadth of particle size dispersity. Our simulations testified to the existence of terminal diameter dispersity, i.e a dispersity above which there can be no amorphous-crystalline phase coexistence. At the terminus, size-dispersity increases from 5.8% to 6.7% for the crystalline phase as pair in...
MRS Proceedings, 2002
Molecular dynamics simulations were used to study the interlayer structure and dynamics of polyst... more Molecular dynamics simulations were used to study the interlayer structure and dynamics of polystyrene (PS) and polystyrene-polyisoprene (PS-PI) block copolymers intercalated in organically modified layered silicates. In the case of PS the polymer chains displace the aliphatic surfactant chains and reside adjacent to the silicate layers. The electrostatic interactions between the aromatic rings on the PS chains and the silicate surface drive the intercalation of the polymer into the host galleries. PI, which lacks such electrostatic interactions, is immiscible (does not intercalate) with the host. There appears to be a minimum number of PS mers for intercalation of PS-PI copolymers to take place. The intercalated copolymer appears to structure inside the host galleries with the PS mers adjacent to the silicate layers and the corresponding PI away from the surface and towards the middle of the gallery. Using the mean square displacements we find that PS is the least mobile species in...
Thesis University of Minnesota 1989 Source Dissertation Abstracts International Volume 50 07 Section B Page 3038, 1989
AIP Conference Proceedings, 2007
We study the conformational transitions of polymer chains near attractive surfaces using Monte Ca... more We study the conformational transitions of polymer chains near attractive surfaces using Monte Carlo simulations on a simple cubic lattice. The shape of the adsorbed chain is studied as a function of the molecular weight, solvent quality and surface energy. The competition between two transitions induced by varying solvent conditions and the existence of an attractive wall results in various shapes of the adsorbate.
AIP Conference Proceedings, 2007
The symposium focuses on the current state-of-the-art in "Computational Methods in Chemical Engin... more The symposium focuses on the current state-of-the-art in "Computational Methods in Chemical Engineering: Physical Chemistry", as they lead to the next generation of methods, approaches, and research foci in the field. This field encompasses a broad range of problems and approaches: ranging from atomistic quantum mechanical calculation of molecules to computer aided modeling and simulation in the development, integration and optimization of industrial processes. Here we limit the symposium's thematic focus on topics that are under the general scope of "Physical Chemistry", in the broader sense, embracing traditional chemical engineering topics (e.g. thermodynamics, solutions, polymers) and looking forward to research directions where the field is expanding (e.g. life sciences, biological engineering).
AIP Conference Proceedings, 2007
We have studied the solid-fluid coexistence for systems of polydisperse soft spheres with near-Ga... more We have studied the solid-fluid coexistence for systems of polydisperse soft spheres with near-Gaussian diameter distributions that interact via an inverse power potential, the softness of which is being tuned. The study employs simulation in the isobaric semigrand ensemble. Gibbs-Duhem integration is used to trace the coexistence pressure as a function of the variance of the imposed activity distribution. Both
We have studied the solid-fluid coexistence for systems of polydisperse soft spheres that interac... more We have studied the solid-fluid coexistence for systems of polydisperse soft spheres that interact via power-law potentials. We employed isobaric semi-grand ensemble simulations. Gibbs-Duhem integration traced the coexistence pressure as a function of the variance of the imposed activity distribution. Fluid-solid coexistence densities were determined to be monotonically increasing functions of the breadth of particle size dispersity. Our simulations testified
Physical Review A, 1987
The statistical uncertainty in the calculation of velocity autocorrelation functions and self-dif... more The statistical uncertainty in the calculation of velocity autocorrelation functions and self-dift'usion coefficients from molecular dynamics is empirically determined from the spread of the moleculardynamics results for an ensemble of macroscopically identical systems. The "experimental" uncertainties of the velocity-autocorrelation-function values at equilibrium and in the presence of flow agree well with theoretical predictions. The uncertainty of the self-diffusion coefficient is found to decrease as the inverse square root of the averaging time.
Makromolekulare Chemie. Macromolecular Symposia, 1993
We have performed molecular dynamics, and lattice Monte Car10 simulations of polymeric melts in t... more We have performed molecular dynamics, and lattice Monte Car10 simulations of polymeric melts in the vicinity of solid surfaces. The structural features of the solid-melt interface were very simple. The interfacial width was comparable to the segment size. Inside this narrow interface the segment density profile was oscillatory. The density oscillations were much less pronounced than those present at solid-atomic liquid interfaces. On a scale much larger than the segment size, chain conformations were found to be identical with those of ideal chains next to a reflective barrier. In particular, the number of surface-segment contacts scaled like the square root of the molecular weight. Extensive molecular dynamics simulations showed that chain desorption times increase with molecular weight but at a rate much slower than the longest relaxation time of Rouse chains. Therefore, sufficiently long chains desorbed almost freely from the surface despite the presence of attractive surface-segment interactions. A study of chain relaxation dynamics c o n f i i e d that the Rouse modes constitute an appropriate set of normal coordinates for chains in the melt interacting with a solid surface. The effect of the surface on mode relaxation was significant. All relaxation processes of chains located within a couple of radii of gyration from the surface were slowed down considerably. This effect, however was approximately the same for fast and slow modes and independent of molecular weight for sufficiently long chains.
We performed lattice MC simulations of single, flexible, self-avoiding chains in bulk solution^*,... more We performed lattice MC simulations of single, flexible, self-avoiding chains in bulk solution^*, or adsorbed onto a surface, under poor solvent conditions. Our simulations spanned a wide range of chain lengths (N=20-10000) and cohesive energies. The chain length dependence of the chain size in poor solvents was characterized by a wide plateau of almost null growth. This plateau was related with the development of the incipient constant density core. The ``volume approximation'' regime and genuine power law dependence (1/3) was not reached even for the longest chains and poorest solvents studied. Sufficiently long chains became more but not fully spherical and underwent a 2^nd order phase transition. Conformations of the adsorbed chains onto attractive surfaces are not controlled by the bulk theta- temperature, but by a new temperature theta' which depends strongly on the interactions with the surface. The adsorption-desorption transition width is determined by the N-dep...
Dense suspensions of multi-arm star polymers are known to exhibit `liquid-like' ordering due ... more Dense suspensions of multi-arm star polymers are known to exhibit `liquid-like' ordering due to their similarities with colloidal particles. Experimental studies reported a counter-intuitive solidification of these suspensions upon increase of the temperature in marginal solvents( M. Kapnistos, D. Vlassopoulos, G. Fytas, K. Mortensen, G. Fleischer, J. Roovers, Phys. Rev. Lett. 85), 4072 (2000).. Our work investigates by MD simulations the temperature induced changes in colloidal superstructure and star dynamics under marginal solvent conditions. Multi-arm star polymers are modeled as soft spheres interacting via a soft, long ranged potential, derived from polymer physics principles. Simulations show a transition towards a ``glassy'' state at a temperature very close to the one reported experimentally. The features of the transition are consistent with those of ideal glass transitions, as described by ideal Mode Coupling Theory^2 M. Kapnistos, D. Vlassopoulos, G. Fytas, K...